There have been various developments in recent years in the use of magnetic-based transducers for sensing torque in shafts. These have the advantage of being realisable in non-contacting assemblies—particularly useful for rotating shafts. The magnetised transducer element is mounted on or integral with the shaft and the sensor arrangement responsive to the torque-dependent magnetic field emanated by the transducer element is disposed out of mechanical contact with the shaft.
One form of known transducer element is that dependent on circumferential or circular magnetisation which forms a closed loop around the axis of the shaft. The magnetisation itself is transverse to the torque axis and forms a closed loop or annulus about the torque axis. Examples of this form of magnetisation applied in an integral portion of a shaft are disclosed in published PCT application WO99/56099. The exterior torque-dependent magnetic field that is sensed lies generally in the axial direction.
More recently, new forms of magnetisation of transducer elements have been developed. These are annular magnetisations about the shaft axis with the direction of magnetisation being in the direction of the shaft axis. Such an annulus of magnetisation produces a toroid of magnetic flux within the shaft and as an extension of the toroid an annulus of flux exterior to the shaft Such magnetisations are referred to as longitudinal in contrast to the above-noted circumferential magnetisation. One form of annular longitudinal magnetisation is referred to a circumferential-sensing longitudinal magnetisation in which the torque-dependent magnetic field that is sensed is a component in the circumferential or tangential direction with respect to the shaft axis. Transducers employing this form of longitudinal magnetisation are disclosed in published PCT application WO01/13081. Reference may also be had to published PCT application WO01/13082. Both these published applications are incorporated herein by reference.
A second form of annular longitudinal magnetiation is that referred to as profile-shift longitudinal magnetisation. This form of magnetisation is the subject of co-pending PCT application PCT/EP01/04077 filed 10th Apr. 2001 which is incorporated herein by reference. A description of profile-shift longitudinal magnetisation is given hereinafter with reference to FIGS. 1-21 and is extracted from PCT/EP01/04077 and FIGS. 4-25 thereof.
In the transducer element disclosed in PCT/EP01/04077 (the '04077 application) the torque-dependent external field emanated by the transducer element has axially-directed and radial field profiles which are axially-shifted as a function of torque. This will be described subsequently and is a surprising result.
The profile is a measurement of the axially-directed or radial component of the exterior field as a function of position in the axial direction. More surprisingly in tests performed on a transducer element produced by the magnetisation process described in the '04077 application, it has been found that there is no detectable circumferential or tangential component: or at least any such component is so weak as to be lost in noise.
The creation of a transducer element having the abovementioned field distribution characteristics is described below. The magnetisation of the element is accomplished by relative rotation about an axis of a shaft in a magnet system generally as illustrated in FIG. 6 of WO01/13081 but with some modification. A greater depth of magnetisation is obtained by using a magnet system comprising permanent magnets forming a horseshoe magnet using stronger magnets than were used previously. More particularly the magnet poles adjacent the shaft have been relatively wide in the axial direction as compared to the spacing or gap between them.
The process for creating the transducer element in a shaft falls broadly into two operations with a third operation that may follow: a magnetic preparation which may be referred to for brevity as de-gaussing or magnetic cleansing; and thereafter a magnetisation procedure. The magnetic preparation (pre-magnetisation procedure) to be described can be summarised as providing a magnetically cleansed part in which the desired magnetic field is then established. The magnetisation procedure may be followed by a post magnetisation procedure somewhat similar to the pre-magnetisation procedure.
It has been found that the exploitation of axial or radial shift profiles is of great utility in magnetic torque transducers. This will be discussed in detail below. However, in practice problems can arise in making accurate torque measurements of a shaft due to sensitivity to axial displacements of the shaft. Both produce an axial shift in the axial or radial profile utilised for measurement.
Another problem which arises in exploiting longitudinally magnetised transducer elements in general is that the annular magnetisation created by relative rotation between a member, such as a shaft, and a magnetic source (permanent or electromagnet) may result in a non-uniformity of magnetisation about the axis of rotation. This is referred to as rotational non-uniformity (RNU) or rotational signal uniformity (RSU).